1. Field of the Invention
The present invention relates to ultraviolet (UV) curable amideimide oligomers and associated ultraviolet curable resinous compositions that may be used as coatings on substrates.
2. The Prior Art
Polyamideimides are utilized in a wide range of commercial applications due to their unique combination of outstanding mechanical properties, excellent thermal stability, high glass transition temperature and good solvent resistance.
Polyamideimides can be made by the polycondensation reaction of an acid anhydride with a diamine. In that reaction, acid groups react with amine groups to produce amide groups, and anhydride groups react with amine groups to produce amic acid groups. The polyamideimide is then obtained by eliminating water from polyamic acid at high temperature (&gt;200.degree. C.) with or without catalysts (see, for example, U.S. Pat. No. 3,984,375). The water evaporation usually generates blisters and defects on the coating. While the blister problem can generally be solved by using a solution of very low solids content so that the polymer is applied as a very thin coating, this means that many passes are needed to build up an adequate coating thickness, which greatly increases the cost of the coating. Also, because the solvent used in the reaction is quite expensive and sometimes costs more than the monomers, a low solids amic acid solution is more expensive than a high solids solution In addition, the removal of solvent during the coating process generates volatile organic compounds emissions in the manufacturing environment.
Polyamideimides can also be made by the polycondensation reaction of an acid anhydride with a diisocyanate in polar solvents (see, for example, U.S. Pat. Nos. 3,518,230; 3,541,038; 3,578,639; 3,562,217; 3,732,186; 3,843,587; 4,069,209; 4,113,706; 4,379,879; 4,420,535; 4,461,805; 4,501,883; 4,505,978; 4,505,980; and, 4,511,681). In this reaction, the acid groups react with the isocyanate groups to produce amide groups and carbon dioxide, and the anhydride groups react with the isocyanate groups to produce imide groups and carbon dioxide. The polymer is preimidized and no volatiles, such as water or carbon dioxide, are evolved during cure. In this method, only the solvent must be evaporated. Thus, the use of diisocyanates to make polyamideimides seems more attractive than the use of amines. However, if the diisocyanate is used in equal or less than equal molar equivalents with the acid anhydride, a low molecular weight polymer and a brittle film results. If excess molar equivalents of diisocyanate are used, the composition has a poor shelf life and its viscosity increases rapidly. The solvent used in the preparation of the polymer still generates the volatile organic compounds emissions.
Therefore, there is a need for a solventless and 100% reactive amideimide oligomers which can be produced without solvent at low temperatures.